1. Field of the Invention
The present invention relates to a process of making a semiconductor device by the use of a silicon substrate or a substrate with a thin film of silicon formed thereon, and the semiconductor device.
2. Description of Related Art
When a semiconductor device is to be produced using a substrate of silicon (which will be referred to as "Si substrate") or a substrate with a thin film of silicon formed thereon (which will be referred to as "thin Si film"), the surface of the substrate is generally cleaned in each step, if necessary. For example, semiconductor devices based on the Si substrates or thin Si films include MOS type field-effect transistors (MOSFET), thin-film transistors (TFT) and so on. When such semiconductor devices are to be produced, the surfaces of the Si substrates or Si thin films to be worked are cleaned, for example, prior to formation of Si oxide films such as gate insulation films and so on.
Methods of cleaning the surfaces of the Si substrate or thin Si film may be mainly classified into four types:
(a) The first method of cleaning by the use of a first acid cleaning solution such as a hydrochloric acid-hydrogen peroxide mixture cleaning solution, or a sulfuric acid-hydrogen peroxide mixture cleaning solution, which may include hydrogen peroxide (H.sub.2 O.sub.2) and hydrochloric acid (HCl), or hydrogen peroxide and sulfuric acid (H.sub.2 SO.sub.4). The first acid cleaning solution will not etch the native oxide film of Si formed on the surface of the Si substrate or thin Si film. PA1 (b) The second method of cleaning by the use of a second acid cleaning solution including hydrogen fluoride (hydrofluoric acid solution) or the like. The second acid cleaning solution will generally etch all the native oxide film on the surface of the Si substrate or thin Si film. PA1 (c) The third method of cleaning by the use of an alkaline cleaning solution such as an ammonia-hydrogen peroxide mixture cleaning solution which may include ammonia (NH.sub.4 OH or NH.sub.3) and hydrogen peroxide (H.sub.2 O.sub.2), or an organic alkaline solution. Such an alkaline cleaning solution will etch the native oxide film on the surface of the Si substrate or thin Si film. PA1 (d) The fourth method of cleaning by the use of any combination of the cleaning solutions described in connection with the aforementioned methods (a) to (c). PA1 (e) In addition, there is a cleaning method disclosed in Japanese Patent Application Laid-Open No. 4-355921, in which after the surface of the Si substrate or thin Si film has been cleaned by an ammonia-hydrogen peroxide mixture cleaning solution, the native oxide film on the top layer thereof is etched and removed by the use of a dilute hydrofluoric acid solution or a dilute hydrogen fluoride-hydrogen peroxide mixture cleaning solution which is a water solution including hydrogen peroxide (H.sub.2 O.sub.2) or the like and the dilute hydrofluoric acid solution. PA1 Method (a): Any metallic impurity contained in the cleaning solution will not be absorbed into the native oxide film formed on the surface of the Si substrate or thin Si film since the cleaning solution is acidic. However, particles on the Si substrate or thin Si film surface will be insufficiently removed since the native oxide film is not etched by the cleaning solution. PA1 Method (b): Silicon of the surface of the Si substrate or thin Si film will be exposed since all the native oxide film is generally etched. It is therefore difficult for the Si substrate or thin Si film surface to maintain its hydrophilic property, resulting in hydrophobic property. The Si substrate or thin Si film surface therefore tends to receive particles and impurities, and the particles in the acidic cleaning solution will easily deposit on the Si substrate or thin Si film surface. To prevent the deposition of the particles, a surfactant may be added into the acid cleaning solution, which may lead to a problem of the waste solution containing the surfactant and the increased operating cost. In addition, the other metals having electronegativities higher than that of the Si material will also deposit on the Si substrate or thin Si film surface. PA1 Method (c): The Si substrate or thin Si film surface tends to be reversely contaminated by the alkaline cleaning solution since the metallic impurities in the alkaline cleaning solution are absorbed by the native oxide film on the Si substrate or thin Si film surface and cannot be sufficiently removed from the alkaline cleaning solution. To avoid the reverse contamination, a chelating agent may be added to the alkaline cleaning solution. It is also required to treat the waste water containing the chelating agent. This also increases the operating cost. PA1 Method (d): The Si substrate or thin Si film surface will be affected by the last used cleaning solution. Thus, the particles or metallic impurities will be insufficiently removed from the Si substrate or thin Si film surface, as in the methods (a) to (c). PA1 Method (e): In the cleaning method disclosed in Japanese Patent Application Laid-Open No. 4-355921, the dilute hydrofluoric acid cleaning solution having a concentration, HF:H.sub.2 O=1:200, or the dilute hydrogen fluoride-hydrogen peroxide mixture cleaning solution having a concentration, HF:H.sub.2 O.sub.2 :H.sub.2 O=1:1:200, is used to etch the native oxide film to a film thickness between 0.1 nm and 0.8 nm. Considering the fact that the concentration of hydrogen fluoride in a hydrofluoric acid solution used in the semiconductor producing factories is normally 50%, the concentration of hydrogen fluoride in the dilute hydrofluoric acid or dilute hydrogen fluoride-hydrogen peroxide mixture cleaning solution is about 2500 ppm. From some experiments, the inventor confirmed that the hydrogen fluoride of such a concentration required 11.4 seconds to remove all the native oxide film on the Si substrate or thin Si film surface. PA1 a first cleaning step of cleaning the substrate with a first cleaning solution, where a native oxide film is formed on the silicon surface; PA1 a first rinsing step of rinsing the substrate wetted with said first cleaning solution with a first rinsing solution; PA1 a second cleaning step of cleaning the substrate including the native oxide film formed thereon with a second cleaning solution including hydrogen fluoride, where a surface layer is removed from the native oxide film; PA1 a second rinsing step of rinsing the substrate wetted with the second cleaning solution with a second rinsing solution, and PA1 time T1 in minutes from the start of the second cleaning step to the start of the second rinsing step being defined by the following relationship: ##EQU1##
However, the aforementioned methods are disadvantageous in the following respects:
In the cleaning step of the actual semiconductor device producing process, the Si substrate or thin Si film is rinsed by the pure water and then dried after it has been cleaned by chemicals. Time required to move the Si substrate or thin Si film from a cleaning bath containing chemicals into a rinsing bath is at least between several seconds and several tens of seconds. When the cleaning solution includes hydrofluoric acid solution, the native oxide film will continue to be etched by the cleaning solution while the Si substrate or thin Si film wetted with the cleaning solution is being moved toward the rinsing bath after being removed from the cleaning bath. Thus, all the native oxide film on the Si substrate or thin Si film will be removed. As a result, the Si substrate or thin Si film surface cannot maintain its hydrophilic property.
Japanese Patent Application Laid-Open No. 63-29516 also discloses a cleaning process in which the native oxide film is left on the substrate surface. However, even if the native oxide film is left on the substrate immersed in the cleaning solution, all native oxide film will be removed by the cleaning solution while the substrate is moved into the rinsing bath after being removed from the cleaning solution for the same reason as described above. Therefore, this cannot reliably prevent the particles from being built up on the substrate. The invention of Japanese Patent Application Laid-Open No. 63-29516 cannot also prevent the metallic impurities from being taken in the substrate.
If the cleaning machine is stopped and thereafter re-started for any reason such as machine trouble while the cleaning solution is on the Si substrate or thin Si film, all the native oxide film may be removed. It is required that the hydrophilic property be maintained even if the set cleaning time is extended.